It consumes ~30-40% of the tokens associated with a project, in my experience, but they seem to be used in a more productive way long-term, as it doesn't need to rehash anything later on if it got covered in planning. That said, I don't pay too close attention to my consumption, as I found that QwenCoder 30B will run on my home desktop PC (48GB RAM/12GB vRAM) in a way that's plenty functional and accomplishes my goals (albeit a little slower than Copilot on most tasks).
In the course of my work, I have found they ask valuable clarifying questions. I don’t care how they do it.
Nearly all of my "agents" are required to ask at least three clarifying questions before they're allowed to do anything (code, write a PRD, write an email newsletter, etc)
Force it to ask one at a time and it's event better, though not as step-function VS if it went off your initial ask.
I think the reason is exactly what you state @7thpower: it takes a lot of thinking to really provide enough context and direction to an LLM, especially (in my opinion) because they're so cheap and require no social capital cost (vs asking a colleague / employee—where if you have them work for a week just to throw away all their work it's a very non-zero cost).
Prompt 1: <define task> Do not write any code yet. Ask any questions you need for clarification now.
Prompt 2: <answer questions> Do not write any code yet. What additional questions do you have?
Reiterate until questions become unimportant.